During the course of drilling an oil well or the like, it is often desirable to expand the diameter of the drill hole below a certain point in the rock formation. An underreamer is a tool lowered in the bore of the hole to do just that. The tool fits rather snugly in the hole. When the desired depth is reached, the underreamer is made to rotate and cutting arms are extended laterally from its body to ream or enlarge the hole. The arms can be retracted for withdrawing the underreamer from the hole.
As the function of the underreamer is to cut rock, it is necessary to employ extremely hard cutting surfaces. Accordingly, tungsten carbide inserts or natural diamonds have been placed on underreamer cutting arms to provide hard cutting surfaces. The high cost of natural diamonds, however, restricts their practical size to a small effective cutting surface.
It is therefore desirable to provide a cutting arm for an underreamer which utilizes synthetic diamonds, which can for a lower cost be made much larger than natural diamonds.
One problem impeding the use of large area synthetic diamonds is the development of excessive heat at the diamond during underreaming. Another problem is accumulation of cuttings on the cutting arm adjacent the diamond cutting surface which restricts the ability of the diamond to cut further material.
Large area synthetic diamond inserts can be mounted in cutter arms of an underreamer, however they must be placed to avoid undue weakening of the arms which could lead to loss of such inserts and to assure cutting of all portions of the rock formation being reamed.
There is need for a cutting arm which incorporates synthetic diamonds of relatively large surface area. The need extends to an underreamer which uses cutting arms including synthetic diamonds and which does not suffer from the limitations described above.